A navigation beacon was a component of an Andorian escape pod which allowed nearby spacecraft to easily detect it. The beacon blinked intermittently, which was visible on the craft's hull, while active.
In December 2153, a navigation beacon was used by an Andorian escape pod containing Starfleet Captain Jonathan Archer. The beacon helped Enterprise NX-01 to find him. (ENT: "Proving Ground")
The term "navigation beacon" comes from the final draft script of "Proving Ground".